The invention is related to sodium transition metal cathode materials for a rechargeable sodium battery. The transition metal is composed of mainly manganese and a minority of bivalent metal dopants such as copper, zinc and nickel.
There are worldwide efforts to limit consumption of fossil fuels by introducing “green cars” like electric vehicles. Currently the trend is to use Li rechargeable batteries for these cars. However, this approach is disputed because it might be environmentally difficult to produce sufficient lithium precursors in a sustainable way.
Lithium is abundant; the content of Li only in seawater is estimated to be 230 billion tons. However, lithium is generally occurring in low concentrations which limits the practically available recourses to about 13 million tons according to a recent survey. These resources are insufficient for a world full of “green cars”. Besides efforts to expand the availability of lithium, for example by extraction from seawater, Li-free rechargeable batteries, such as sodium rechargeable batteries, receive renewed interest.
In many aspects sodium behaves electrochemically similar to lithium. The theoretical possibility of sodium ion batteries was disclosed in the late 70ties. The successful reversible intercalation and de-intercalation of sodium ions in a cathode material (NaxCoO2) has been described already in 1981 by Delmas, who showed data of half cells (Na-cathode and sodium metal anode). However, as in lithium batteries, sodium metal cannot be used as anode material in commercial batteries due to dendrite formation, so an anode material adapted to sodium ion batteries is required. In 1999, a group at Dalhousie University developed hard carbon as Na intercalation anode material. Since that time the feasibility of Na batteries is in principle accepted, however, in the shade of the ever-increasing Li rechargeable battery world only relatively few efforts were assigned to develop practical sodium intercalation batteries.
Sodium ion batteries are thus in their early stages of technological development. It is already clear now that layered sodium ion cathodes can be cheap because they need less cobalt or can even be cobalt free. The possibility to avoid cobalt is related to basic crystal-physical properties. Sodium bronzes are perfectly layered without cobalt, whereas in LiMO2, Co is important to stabilize the layered crystal structure, and to prevent cation rearrangement (migration of transition metal to lithium sites) during cycling.
Some prior work about manganese based Na containing cathode materials exists: patent applications US 2010/0248040 A1 and US 2011/0200879 A1 by Sanyo teach to apply cathodes which contain sodium as well as lithium for Li ion batteries (not sodium ion) batteries. US 2010/0248040 A1 describes NaaLibMxO2 where M is selected from Mn, Fe, Co, Ni. But the cathodes were only used in Li ion batteries. In “P2-type Nax[Fe1/2Mn1/2]O2 made from earth-abundant elements for rechargeable Na batteries” by Yabuuchi et al. (in Nature Materials 11 (2012), 512-517), a cathode material NaxFe1/2Mn1/2O2 has been disclosed. This cathode material does not contain scarce or toxic elements, and it shows high reversible capacity in Na ion batteries. The cathode material is a true intercalation material. The Fe—Mn-oxide host structure remains intact during sodium de-intercalation and re-insertion. Sodium ion batteries containing high capacity sodium cathodes like NaxFe1/2Mn1/2O2 and anodes like hard carbon increasingly will be able to challenge the dominance of Li ion batteries, especially for green cars. However, in the known layered sodium metal oxides the capacity loss during electrochemical cycling is not sufficiently low compared to LiMO2. Since the ionic radius of Na+ is higher than Li+, the volume change of unit cell during (de-)intercalation is more important, resulting in the continuous collapse of the layered structure during cycling. In patent application WO2014/132174, Na2/3MnO2 based materials with a low-valent state dopant such as Li, Mg and Ni achieved a high capacity. Recently, copper doped materials, Na2/3Cu1/3Mn2/3O2 were reported by Xu et al. (in Chin. Phys. B 23 (2014) 118202) and by Mason et al. (in ECS Electrochemistry Letters 4 (2015) A41-A44) claiming the reversible change of Cu2+/Cu3+ redox couple. Moreover, co-doping with Cu and Fe was applied by Li et al. (in Adv. Sei. 2 (2015) 1500031) and a series of co-doping with Cu, Fe, Al, Mg, Ti, Co, Ni and Zn was reported in CN104617288. All copper containing Mn based materials show low capacities of less than 100 mAh/g because a large amount of Cu doping sacrifices Mn's reaction of the Mn3+/Mn4+ redox couple. In Energy & Environmental Science, vol. 4, No 4, p. 1387, J. Billaud discloses Na0.62Mn1-xMgxO2 (0≤x≤0.2) cathodes for sodium-ion batteries. P2-type Na0.67Mn0.65Fe0.2Ni015O2cathode material is disclosed in Electrochimica Acta, vol. 116,2013, pages 300-305.
While all materials in above mentioned prior art have a hexagonal structure, NaxMnO2 can have an orthorhombic structure which is a distorted P2-type layered structure. P2 orthorhombic layered Na2/3Mn2/3O2 was disclosed by Jens et al. (in Solid State Ionics 126 (1999) 3-24) and by Stoyanova et al. (in J. Solid State Chem. 183 (2010) 1372) without specifying however any electrochemical property. Both literatures pointed out the difficulty to obtain a pure orthorhombic phase of NaxMnO2 because mixed phases of hexagonal and orthorhombic were obtained.
It remains an open question if sodium rechargeable battery technology will industrially succeed, by allowing a different design when compared to lithium ion battery technology. It is however clear that the rechargeable sodium battery technology has the potential to replace lithium technology if cheap sodium cathode materials with high capacity allow to achieve a high safety and a good calendar life at low cost. This problem is addressed in the present invention.